Gradient on horizontal ducting

[omgboy2]

Hi, I'm installing a new kitchen cooker extractor. There is no previous ducting.

I plan to run rectangular ducting up through the unit and across the top of the cabinet to the outside wall (100mm). The carpenter has already drilled the hole in the wall, but he drilled it slightly above the cabinet top.

This means there will be a slight gradient upwards towards the wall vent. Is this okay for regulations?

Alternatively I could put a small upward adapter near the outlet and keep the rest of the ducting horizontal.

Please advise. It needs to be signed off.

 

[jj4091]

What do you mean by "slightly" & is the hole through the wall level?

 

[omgboy2]

The pipe goes up vertically from the canopy and then goes across horizontally roughly 2m above the cabinets to the external wall. it slopes up 25mm over the 2m to the vent.

 

[omgboy2]

Also it says the cooker hood needs 120 mm ducting, the hole in the wall is for a 100mm outlet. Should I use 100mm all the way or use 120mm and all the way and then reduce at the wall conenction point

 

[jj4091]

If you are using flat ducting on top of the cabinets just mount it on a piece of flat timber that is 25mm thick to level it up to the hole, then use the flat to round coupling to join up to pipe through the wall. The pipe & hole through the wall being level or falling down to the outside is probably more important in order to stop rain water running back if any gets in. Personally I would not worry about the ducting size, but others do.

 

[Deleted member 174758]

The smaller the duct, the greater the resistance to airflow, so the hotter your fan motor will run. Reducing the 120mm ductwork at the wall end will still have the effect of reducing the airflow of the entire duct to that which a 100mm pipe can carry. In reality I doubt it will make that much difference, though, as the change isn't huge

AFAIK I don't think there are any regulations about direction or amount of fall on domestic kitchen extraction (there might be nowadays, but somehow I really doubt it, after all who is going to police the regs?), but it obviously makes sense to have your pipe running slightly downhill towards the outside to ensure that when steamy air cools down in the duct run (as it does), any condensation does not either pool at a low spot (becoming a potential breeding point for bacteria and mosquitos - again more of an issue in a professional kitchen), or run backwards and drip into your pots and pans on the hob. I'd suggest that you may need to consider some form of weep hole at the lowest point, together with a drain, depending on how much steaming (rice veg, etc) you do, although I'm not sure how I'd make one up (possibly a job for 20mm PVCu conduit and solvent cement?)

 

[Deleted member 174758]

The pipe & hole through the wall being level or falling down to the outside is probably more important in order to stop rain water running back if any gets in.

But with most vents directing exhaust air flow downwards (and even closing when there is no flow) that surely isn't a big issue

 

[jj4091]

But with most vents directing exhaust air flow downwards (and even closing when there is no flow) that surely isn't a big issue

Vents fall off & get broken.

 

[Deleted member 174758]

Not that often, they don't